Several methods exist to transmit light information over optical fiber links. High end/high performance systems generally use lasers as sources of coherent beams of light, along with some form of an optical modulator. Electrical signals control the optical modulator to modulate light: such modulators can affect the amplitude, phase, and/or frequency of the transmitted light stream. Lower end/lower rate systems such as first generation Ethernet Passive Optical Network (EPON) or Gigabit Passive Optical Network (GPON), on the other hand, directly modulate the current fed to a laser diode, resulting in a modulated light stream.
Direct modulation may be cost effective as it does not require expensive modulators or light sources. However, this method usually imposes a limit on the maximum modulation rates at which a link can be operated. Low-cost lasers are typically encapsulated into lead packages which exhibit high inductance that can limit the bandwidth of laser diode circuits. Lower bandwidths lead to higher inter-symbol interference (ISI) and complicate data recovery. ISI results from the unsettled circuit response to a change in symbol (or bit for 1-bit symbols) values. Unsettled responses can cumulate and lead to blurring that can makes information recovery tedious.
Also, laser diodes can produce spurious light emissions at the onset and cessation of coherent light emission, known as “chirping”, which further impairs data recovery. The “chirp effect” is a high frequency ringing of the emitted light's amplitude that occurs during the turn-on phase of a laser. It is a non-linear behavior that cannot be compensated and which closes the optical eye. The shorter the unit interval (UI) of a bit stream, the more pronounced the impact this effect has on the eye quality, because the chirp is of fixed duration and thus does not scale with the bit duration. The extent and magnitude of the chirp can be minimized by ensuring the laser diode is properly biased.
A laser diode driver (LDD) is used to bias a laser diode and to modulate its current. The laser diode load driven by the LDD often presents large inductive characteristics that result in high voltage transients during modulation. The inductive characteristics are commonly caused by three effects: (1) the low-cost packaging techniques used for the discrete laser diode, which introduces parasitic inductors due to the package leads and bond wires; (2) inductors and components involved in circuit bandwidth extension techniques; and (3) inductors used isolate bias current from modulation currents.
In this kind of environment, potential peaks of several volts can be observed during transients induced by changing the bias and modulation currents of the laser diode.
Traditionally, LDDs come as discrete components and are placed on printed circuit boards. They are built using robust and proven semiconductor technologies (typically bipolar) adapted for analog design, and can sustain the large voltages required to drive laser diodes. Normally, LDDs interface with digital integrated circuit (IC) controllers, typically built using complementary metal-oxide-semiconductor (CMOS) technology. The controllers provide the traffic payload which is converted by an LDD into a signal suitable to operate a laser diode. LDDs therefore sit between the IC controller and laser diodes. The digital payload received by LDDs consists of a serialized bit stream that takes the form of an electrical voltage signal, since proper data serialization must take place before reaching the laser driver. Communication between the controller and the LDD uses standard signaling schemes, for example, low-voltage positive emitter-coupled logic (LV-PECL) to enforce a clear definition of electrical levels and timing specifications. This ensures the information transmitted to the LDD is correctly interpreted.
Recently, there has been interest in building LDDs through mainstream CMOS processes, as the LDDs can then be integrated into the digital IC controller, resulting in reduced cost, power and area. Modern CMOS processes are, however, not optimized for analog design, and cannot tolerate the large voltage transients required to drive a laser diode due to the transistors' small feature sizes. These large voltage transients cause electrical overstress (EOS) conditions on the circuit and premature wear of the output transistors, resulting in reduced performance and lifetime.
It is, therefore, desirable to provide a method to reduce the transient voltage peaks generated by laser diode circuits, making possible the integration of LDDs in mainstream CMOS technologies.